Docs/02 schemas

[flsh86]

Schema derivation was explained (differently) in three structured-outputs pages and the agent-tools page. This PR gives it a single home and untangles the OpenAI structured-outputs page.

• Add a shared page other/json-schemas.md: what problem a JSON Schema solves → the easiest way (derives from a case class) → customising the derived schema (Tapir) → building one manually. All provider pages link here instead of re-explaining derivation
• Restructure openai/structured-outputs.md to a clear flow (typed createChatCompletionAs[T] → strict-mode normalization → lower-level ResponseFormat.JsonSchema) and extract the flight-booking function-tool walkthrough into a new dedicated openai/tool-calling.md — OpenAI previously was the only provider without a tool-calling page
• Align claude/structured-outputs.md with the same flow (the section titled "Basic" was actually the lower-level path) and compile-check its manual-schema snippet
• Link the Gemini structured-outputs and tool-calling pages to the shared schema page
• Give the tool-definition preamble in agents/tools.md a proper heading and compile-check its example